<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>onchange事件</title>
</head>
<body>
  <label>
    选择一项
    <select onchange="alert(this.value)">
      <option value="value1">1</option>
      <option value="value2">2</option>
      <option value="value3">3</option>
      <option value="value4">4</option>
    </select>
  </label>
</body>
</html>